1. Field of the Invention
The present invention relates to a multilayer ceramic capacitor and a manufacturing method thereof. In particular, the invention relates to a small, high capacity multilayer ceramic capacitor used in high-performance electronic equipment, such as personal computers and cellular phones, which comprises a dielectric layer and an internal electrode layer that are extremely thin and alternately laminated, thereby exhibiting excellent reliability such as capacity temperature characteristic and high-temperature load lifetime, as well as a manufacturing method of the same.
2. Description of Related Art
Recently, along with miniaturization and high performance of electronic equipment, a multilayer ceramic capacitor used therein demands a smaller size and a higher capacity. For the purpose of this, there is a tendency to increase the number of laminations of dielectric layers and internal electrode layers, and also thin the dielectric layer itself. Additionally, attempts are made to increase reliability such as capacity temperature characteristic and high-temperature load lifetime, as the characteristic of a multilayer ceramic capacitor.
As such a multilayer ceramic capacitor, ones disclosed in the following patent documents are known.
First, in a multilayer ceramic capacitor disclosed in Japanese Patent Publication Laid-Open No. 2001-230149, a dielectric porcelain is prepared in such a method of previously calcining BaTiO3 and MgO, and then adding various oxides of rare earth elements, acceptor type elements, etc. to this calcined powder. There is described that with this two-stage mixing method, even after firing, the presence of the MgO previously allowed to go into solid solution can suppress diffusion of the subsequently added various oxides of rare earth elements and acceptor type elements, etc. into the BaTiO3 crystal particles, thereby obtaining the above-mentioned desired characteristics.
Japanese Patent Publication Laid-Open No. 9-241075 describes that a dielectric porcelain is composed of two or more types of crystal particles having a mean particle size of 0.1 to 0.3 μm, and having different capacity temperature characteristics, thereby obtaining a multilayer ceramic capacitor that has flat capacity temperature characteristic and excellent DC bias characteristic.
In accordance with this publication, it is difficult to form crystal particles, being called “core shell structure,” which realize flat capacity temperature characteristic and excellent DC bias characteristic when the particle size is not more than 1 μm in dielectric particles comprising mainly BaTiO3. Consequently, dielectric particles having a particle size of not more than 1 μm should be subjected to further grain refinement so as to suppress dielectric activity, thereby obtaining flat capacity temperature characteristic and excellent DC bias characteristic, as a whole of dielectric porcelain.
Japanese Patent Publication Laid-Open No. 2000-58378 describes that the use of Ba1-xCaxTiO3 in which Ca is substituted in part for Ba of BaTiO3 constituting dielectric porcelain also provides flat capacity temperature characteristic and excellent DC bias characteristic.
In the multilayer ceramic capacitor disclosed in the Japanese Patent Publication Laid-Open No. 2001-230149, the employment of a preliminary step of previously mixing and calcining BaTiO3 and MgO enables to increase relative dielectric constant and also satisfy B characteristic in capacity temperature characteristic (−25° C. to 85° C. in temperature range; and within ±10% in capacity change rate), while capacity temperature characteristic does not satisfy a wide temperature range, i.e., X7R (−55° C. to 125° C. in temperature range; and within ±15% in capacity change rate).
In the dielectric porcelain described in the Japanese Patent Publication Laid-Open No. 9-241075, its relative dielectric constant can be increased up to about 2100, due to the grain refinement of dielectric particles.
In the Ba1-xCaxTiO3 described in the Japanese Patent Publication Laid-Open No. 2000-58378, the relative dielectric constant drop due to the substitution of Ca is too large to achieve a relative dielectric constant of more than 2000.
Especially, the capacitors provided with the dielectric layer as described in the foregoing respective patent documents exhibit low relative dielectric constant in the range of alternating field intensity of 0.002 to 1 Vrms/μm.